IV ON PLANT LIFE 131 
same stock. They have been long known to 
children and gardeners, who call them thrum- 
eyed and pin-eyed. Mr. Darwin was the 
first to explain the significance of this curious 
difference. It cost him several years of 
patient labour, but when once pointed out it 
is sufficiently obvious. An insect thrusting its 
x 250 
Fig. 12. Fig. 13. 
Flower and Pollen of Primrose 
proboscis down a primrose of the long-styled 
form (Fig. 12) would dust its proboscis at a 
part (a) which, when it visited a short-styled 
- flower (Fig. 13), would come just opposite 
the head of the pistil (st), and could not fail 
to deposit some of the-pollen on the stigma. 
Conversely, an insect visiting a short-styled 
plant would dust its proboscis at a part farther 
